Recently, the reduction of a pitch of integrated circuits has been advancing due to the demand of miniaturization of information processing equipment and high-speed performance thereof. In accordance with this, the actual mounting system is changing from a through-hole type to a surface mounting type and the arrangement of terminals is changing from a peripheral arrangement to an array arrangement. Furthermore, of such array arrangements, a ball grid array (BGA) in which solder balls are arranged in an array-like arrangement on a surface of a package is becoming a main type of a package on which the terminals may be mounted on the surface.
In the case where a socket is used between a BGA type integrated circuit and a substrate, in some cases, the socket may be actually mounted on the substrate for interchanging the integrated circuits in addition to the case where the integrated circuits are subjected to a burn-in test or a high frequency test by using a test socket. In the socket, it is necessary to positively contact all the contacts with both terminals of the integrated circuits and terminals formed on the substrate to keep a sufficient electrical conductivity.
On the other hand, it is required to reduce the inductance with respect to a higher speed clock as the density or the operational speed of the integrated circuit is increased. In order to meet this requirement, it is necessary to make a path length of current in contacts (interval between contact portions), which are interposed between the terminals of the integrated circuit and the terminals of the substrate, as small as possible.
Conventionally, for example, a socket of a semiconductor package having a plurality of solder balls arranged in an array is disclosed in Japanese Patent Application Laid-Open No. Hei 8-222335.
The socket 300 of the publication is provided with a contact 305 as shown in FIGS. 12 to 16. This contact 305 is formed by punching a metal plate out and is provided with a base body 302 having side walls 301 on both sides and having a substantially U-shaped cross-section, a substantially C-shaped contact piece 303 having elasticity, projecting toward the side walls 301 from a lower portion of the body 302 and extending above the body 302, a contact portion 303a provided in the vicinity of a tip end of the contact piece to be brought into contact with a solder ball S of an IC package 400, and a contact portion 304 projecting from the lower end portion of the body 302 in a direction opposite to that of the contact piece 303.
In this test socket, in order that the solder ball S pressingly contacts the contact portion 303a, the contact piece 303 is formed into a leaf spring having an arcuate shape. For this reason, it is difficult to shorten the length of an electric path from the contact portion 303a to the contact portion 304. Thus, it is impossible to decrease the self-inductance. This causes a problem that the test or evaluation of the semiconductor package could not be performed with high precision in a high frequency range. The same problem arises also in the case of a socket comprising a contact in which, as shown in FIG. 16, contact portions are formed at its respective upper and lower ends, and a spring is provided between the contact portions. Further, since the contact is brought into contact with the solder ball only at one point, contact precision is somewhat lower as compared with the case where it contacts with the solder ball at two points.
As a technique for solving such a problem, a socket shown in FIG. 16 is known. A contact 310 formed substantially into a U-shape is press-fit and fixed to the socket body 311, and the portion of upright piece 313 except for a fixture region 312 can be elastically deformed. Thus, when the contact portions 314 are brought into contact with the solder ball 315, the upright pieces 313 are elastically deformed so as to avoid damaging the solder ball 315. Furthermore, a height (thickness) of the socket can also be reduced.
However, also in this technique, there are problems to be solved as described below. Namely, these are problems associated with the requirement of further reduction of height of the socket. In the socket shown in FIG. 16, the length of the upright piece 313 that may be elastically deformed is shortened due to the existence of the fixture region 312. For this reason, there is a problem that it is difficult to further reduce the height thereof. Furthermore, since a maximum dimension of the contact 310 is elongated, it is impossible to further reduce the inductance.
In particular with respect to the reduction in height of the socket, if the portion of the upright piece 313 that may be elastically deformed is shortened due to the further reduction in height of the socket, the elasticity is degraded so that the load imposed on the solder ball 315 is increased to cause the problem of damaging the solder ball 315.
As a technique for reducing the height of the socket, there is a contact sheet described in Japanese Patent Application Laid-Open No. 2001-167857, in which a contact spring is formed by a plurality of cantilevers having spherical terminal holding portions.
In the contact sheet described in the publication, since all the cantilevers are arranged on a planar surface with respect to the sheet, it is possible to ensure the advantage of the further reduction in height but there is a problem that the arrangement thereof at a high density is impossible. Namely, since the plurality of cantilevers are laid horizontally on the planar surface, the area occupied by one contact spring is increased. As a result, the high-density arrangement is impossible.